Synergistic effects when using mixtures of optical brighteners to brighten polyester (PES) textile materials are known and are widely used in the textile industry in order to achieve effect enhancements and cost savings. The effect of the synergy is observed following textile application, i.e. by applying the brightener mixture by the exhaust process or pad thermosol process. A review of the literature on interpretation of synergism is given in Melliand Textilberichte (65) 1984, 327-329.
The synergistic mixtures employed to date only develop their effect when the individual components of the optical brighteners for PES possess nonionic structures.